1. Field of the Invention
The present invention relates to an electronic circuit, and in particular, relates to an electronic circuit having a differential amplifier circuit receiving an input signal and a reference signal and generating a reference signal by smoothing the input signal.
2. Description of the Related Art
Recently, a PON (Passive Optical Network) type FTTH (Fiber to the home) is widely spread. There is a demand for receiving a variable amplitude optical signal from each home and amplifying the optical signal as electrical signal, with respect to the system.
FIG. 1 illustrates a block diagram showing an electronic circuit 100 that is a receiving circuit used for a PON type optical communication disclosed in Japanese Patent Application Publication No. 2005-223638. A light-receiving element such as a PD (photo diode) 10 receives an optical signal and outputs an electrical current. A trans-impedance amplifier (TIA) 12 converts the electrical signal from the PD 10 into a voltage signal. The electronic circuit 100 has differential amplifier circuits 14 and 16 and a reference signal generation circuit 18. The differential amplifier circuit 14 differentially amplifies an input signal VA and a reference signal Vref, the input signal VA being an input signal of the differential amplifier circuit 14 and fed into the electronic circuit 100, the reference signal Vref being generated by the reference signal generation circuit 18 from the input signal VA. The differential amplifier circuit 16 amplifies and saturates the output of the differential amplifier circuit 14, and outputs differential output signals Vout and VoutB. The differential amplifier circuit 16 outputs a high level as the output signal Vout and outputs a low level as the output signal VoutB when the input signal VA is higher than the reference signal Vref. The differential amplifier circuit 16 outputs a low level as the output signal Vout and outputs a high level as the output signal VoutB when the input signal VA is lower than the reference signal Vref. The receiving circuit used for the PON type optical communication selects a signal related to an average of input signals as a reference signal, compares the reference signal and the input signals, and outputs a high level and a low level, because amplitude of an optical signal is not constant.